On your first 1/2 DMD route you can get food/water/etc. in Clayton after Diablo and in Livermore after Morgan Territory. There are water and bathrooms in a park at the top of Morgan Territory. This loop is very easy to resupply on. The route will take you right past stores and restaurants in Clayton and Livermore.
The Mines Rd. / Hamilton Route is trickier. You pretty much get the Junction Cafe at Mile 30 and the Lick Observatory at the top of Hamilton. You have to check the hours on both when you plan your ride.
Junction Cafe sells hot food and also has candy bars etc. with a cooler case of water, gatorade etc.
Lick Observatory has vending machines that sell Cliff Bars, candy, drinks etc. You can get change in the gift shop.
There is also a 7-11 right before the climb up Sierra Rd at around Mile 72.
Go for it! (on these loops at least)
They are a great way to gauge if you want to take a shot at the big one.
I rode these roads a bunch over the last couple years.
I used them last spring as home field advantage to train on for my successful 2011 DMD effort.
Good luck!