I have just moved home, and now most of my run training will be on trails.For 70.3 distance racing, is there a down side to trail running 90% of the time. The terrain is fairly hilly
No real downside. Slightly increased risk of some types of injury (e.g. rolled ankles), but reduced risk of some overuse injury because it’s generally easier on the body. So that balances out. It’ll be slightly slower running overall. So for the remaining 10% you might want to do speedwork to keep the cadence up.
as said above, I find that my risk of injury is a little higher with trail running.
having said that, it’s a great opportunity to do short bursts of high intensity work if the terrain is hilly, as yours seems to be.
plus, it’s refreshing to get off the roads every now & then. can be a good change to your workouts.
Works for me. I’ve been battling plantar fasciitis, and trail running is much easier on my feet than road running. I’ve run once on the road since mid September, and that was a 4mi Turkey trot. Was able to hold roughtly a 6:08 pace based ONLY on trail running where my average pace almost never dips below 8 min miles.
I got into trail running two years ago as I got into the Ultra distance thing. The strongest I have ever been as a runner was during this time. If you find trails with plenty of hills, you’ll get stronger, faster and become a better overall runner. Sure, there is the risk of injury, ankles mainly, but I found my ankles got so strong because of the uneven terrain you’re constantly running on. Also, it’s so much easier on the muscles and joints. I could go run 25 miles on trails one day, and go run 10+ the next with no issues. I run 25 miles on the road, the next day I’m usually doing something to recover that doesn’t include running.
Be sure you try to transition into trail running over a few weeks. I moved to the Black Hills National Forest and went full throttle into the trails, and my ankles really paid for it. I never got injured, but certainly came close. All it took as a couple weeks of doing easier trails and more road running and another couple weeks to ease back onto the trails.
Speed work is tricky on the trails but other than that I think it will be beneficial. I do think that you should do some longer runs with race pace efforts on the road leading up to the race to get a feel for it. Trail paces will be slower and when you get back on the road it might feel like you are going fast when you really aren’t 
I have been trail running for 99% of the time the last few years. Have not had any issues during run races on the streets.
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Other than specifity if your races take place on roads, not much downside. I am a rookie runner (cyclist back ground) and started a similar thread about a month ago. I have noticed i’m less prone to ankle roll now I’ve been trail running a few months and right from the start the recovery time was much less. I would rotate some of your sessions from trail to run and vice versa periodically. So far I think it’s been instrumental in getting my mileage up rapidly. Only up to 3hrs/week (+cycling) but am not getting any overuse injuries at all.
What kind of trails? Crushed stone paths that are smooth and wide, or tight turning gnarly singletrack? There’s a big difference. If it’s smooth trails, you’ll be fine. If it’s technical singletrack then you’ll want to add miles on the road. Also depends on how fast you want to run a 13.1 in. The faster you want to go, the more the surface matters.
I got into trail running two years ago as I got into the Ultra distance thing. The strongest I have ever been as a runner was during this time. If you find trails with plenty of hills, you’ll get stronger, faster and become a better overall runner. Sure, there is the risk of injury, ankles mainly, but I found my ankles got so strong because of the uneven terrain you’re constantly running on. Also, it’s so much easier on the muscles and joints. I could go run 25 miles on trails one day, and go run 10+ the next with no issues. I run 25 miles on the road, the next day I’m usually doing something to recover that doesn’t include running.
^^^^^^^ This