I did this last year, nearly all my rides were on the trainer. I'd say to be sure to integrate intervals instead of just focusing on steady state rides. I focused too much on steady effort versus intervals and didn't really get any stronger. (Did 3 70.3 and 1 IM distance last year, same for this year)

This year, I've been doing more interval work and have become a much stronger rider, thus when doing steady state rides, I am doing so at a higher power output at same effort level.

Just my .02, Trainer Road and Zwift both have workouts that can accommodate this, go with whichever you prefer, I use Zwift.