Short answer: for your first trail run, pick something shorter and flatter than your usual road runs, judge it by time on your feet and how much climbing it involves rather than distance, and choose a route you can cut short easily. Expect to run slower than on road, plan to walk the steep bits, and build up from there once you know how trail feels.
The leap from road running to trail running trips a lot of people up, and almost always for the same reason. They pick a route the way they'd pick a road run, by distance, head out at their normal pace, and discover about twenty minutes in that trail is a completely different sport. Here's how to choose a first route that leaves you wanting to go again, rather than one that leaves you sitting on a rock questioning your life choices.
Is trail running just road running on a trail?
No, and this is the single most useful thing to know before you pick a route.
On the road, the ground is flat, predictable and fast. On a trail, you're dealing with climbs, descents, mud, roots, rocks and the occasional gate, and all of it slows you down and asks more of your legs. The same distance will take you longer and feel harder. None of that is a problem once you expect it. It only catches you out when you plan your first trail run as if it were a road 5k and then wonder why you're wrecked halfway round.
How should I judge how hard a route is?
Look at three things together: how long it'll take, how much climbing it packs in, and how rough the ground is.
Distance on its own tells you almost nothing on a trail. Five flat kilometres on a canal towpath is an easy jog. Five kilometres up and over a hill on rocky ground is a proper outing. When you're sizing up a route, the honest signal is the climb, so check the total ascent and the elevation profile before anything else. After that comes the surface. A smooth, firm path runs almost like road, while rooty singletrack or a boggy moor demands constant attention and a lot more effort for the same distance.
How far should my first trail run be?
Shorter than your road runs, and measured in time rather than miles.
A good first target is something you'll comfortably finish in thirty to fifty minutes on your feet, including walking breaks. If you happily run 5k on the road, a first trail outing of around 3 to 4k with gentle climbing will feel about right, and you'll likely be out for a similar length of time because trail is slower. The aim isn't to prove anything. It's to find out how trail feels while you've still got plenty left in the tank.
How do I match a route to my current fitness?
Start from what you already do comfortably, then scale it down for the trail and leave yourself an escape route.
Take a distance and a hilliness you know you can manage on the road without it becoming a slog. Trim the distance back, because everything takes more effort off-road, and be cautious with the climbing in particular, since a route with a lot of ascent will test you far more than its distance suggests. Then, and this is the part most beginners skip, pick a route you can cut short. A loop that passes back near the start, or an out-and-back you can turn around on at any point, means that if your legs have had enough you stop early rather than grinding out a route that's beaten you. Being able to bail is what lets you aim slightly ambitiously without it going wrong.
Walking the hills isn't cheating
Worth saying plainly, because it quietly stops people enjoying trail running: walking the steep climbs is completely normal, and almost everyone does it.
Even experienced trail and fell runners walk the sharp ascents, because on steep ground a fast walk is barely slower than a run and saves a huge amount of energy. Letting go of the idea that you have to run every step is what turns trail running from an exhausting ordeal into something you'd choose to do on a Sunday morning. Run the flats and the downs, walk the ups, and enjoy the bit at the top where you get to look at the view you earned.
Check the route before you go
A couple of minutes looking at a route beforehand saves most first-run surprises.
Have a proper look at the elevation profile so a wall of hill doesn't ambush you, check the surface so you know whether you're in for smooth track or technical ground, and if other people's recordings follow the same line, take it as a good sign the route actually runs. It's also worth noting where your bail-out points are, so you already know your options if you need them. There's a fuller walk-through of sizing up any route in how to plan a walking or running route, and more on tracking down good trails in how to find new trail running routes near you.
Before you set off
Save your route and the map around it for offline before you leave, so a dropped signal out on the trail doesn't matter (there's more on that in do you actually need offline maps for hiking?). Start with a charged phone, carry water for anything beyond a short loop, and tell someone where you're going. Then build gradually. Once you've got a feel for how trail treats your legs, looking back at what you actually ran is the quickest way to judge the next one, which is what what your route data can tell you after a walk or run is about.
Get the route to match where your fitness actually is today, not where you'd like it to be, and a first trail run is genuinely good fun. LaceLock is being built to make that easy. Draw your route or start from one that suits, save it and the map for offline, follow your position live, and have a look back at how it went once you're home.