I'd sit with my legs propped up somehow, legs straight. Stretch the hamstrings a bit, put some tension on them. I found it helps keep them loose when they recover. I have very tight hamstrings so even a little more flex is noticeable. At my old office I'd just recruit an extra chair and put my feet up. Since shoes weren't a requirement it wasn't a big deal. Not sure how formal your office is or how dangerous/dirty the floor is there (i.e. if you work in a metal shop I wouldn't shed my shoes too quickly).
Keep your hamstrings really warm, it helps recovery if you have good blood flow. Heat pad or whatever.
Eat protein. Drink fluids to flush out waste.
Spin on the bike. If you're doing a 100 mile race, you can probably go a couple hours easy and not kill yourself. Getting warmed up and loose and all that will help your legs feel a lot better. Dress warmly, esp your legs.
Take some anti inflamatory (Advil? Tylenol? I dunno, don't take them very often).
A very good MD told me to elevate injured limbs so blood flows away from them. This helps speed recovery as waste and old blood doesn't tend to stay pooled in your limbs. Your hamstrings are injured in a sense so this advice should apply. Since your legs are normally lower than not, keeping them as high as possible would be good.
I've done all of the above. I found the best thing for recovery was to get older. Seems now I can ride much harder and longer (3-4 hours) and still be good the next day. 20 years ago I was dead to the world for 2-3 days after one hard day. Can't explain it.
Good luck with the race. Jeepers, 100 miles.
cdr