Couple years ago I signed up for a local 50k to be run on a very difficult course in terms of roots, rocks, elevation changes, etc. Had probably 40+ road marathons under my belt including Boston. Trained for two to three months on trails, mainly doing 45-60 minute tempo runs during the week and doing a long slow trail run on the weekends (2-4 hours). Was also cycling 4x a week.
No speedwork, no concern for paces at all, and not really looking at my watch except to run for time. Enjoyed not having the grind of a road marathon buildup where every run is done at a certain pace with tempo at 10k or LT runs of an hour, or MP runs, etc. Also loved not having to worry about cars.
Did the trail race, was about the hardest thing I’ve ever done. One week before the race a buddy asked me to do a road 5k with him, and I ran a huge PR in that race. Two weeks after the 50k a local running store gave me a free entry to our local road marathon and I also came close to a PR that day too.
I believe the trails helped to shorten my stride, build up ankle strength and foot strength, and my lower body and core became more durable. I also found running on the roads after my trail buildup to be more fun and exciting because I wasn’t as concerned about falling. Unfortunately, at least once a long trail run I drag my feet and down I go, sometimes hard. Now, at almost 50, I’m a bit more cautious when trail running because my ribs seem to bear the brunt of most falls which causes me to lose training time nursing them back to health. All that said, I’m a firm believer that the road runner can pull in some trail runs to make them a better runner.