WW - just make it one of your goals to do it more outside. Maybe you'll end up being an "athletic freak"
I have a lot of trouble pacing myself too (on treadmill, in a pickup basketball game, running outside, eating lunch, lol) and I think it will get easier the more familiar you are with different trails and do it more frequently. Practice makes competence.
I'm mainly so fired up simply b/c those pictures are so awesome. It's not like you're running around a trailer park :P.
HIIT on a trail (or anything on a trail obv) is going to feel a lot different than HIIT on a treadmill, so just listen to how you feel and how beat you are rather than the time you record for each sprint or w/e.