I do this run a lot my parents live near Elveden. To be frank there are very few places to stop and actually from the M25/M11 it is only going to take you about 1 hour 20 minutes. If you really wanted to stop probably Newmarket is the best bet having a run on the Heath (only after midday as horses being trained prior to then) and something to eat and drink in Waitrose cafe. Then it's about 20 minutes from there.
We live on M25 and my mum also lives near Elvedon and the worst bit is through the forest and there is nowhere to stop along that road. Just before the forest is Barton Mills which I think have a few pub/restaurants but we never bother to stop as Lonecat says its only hour and half unless we get stuck in traffic through the forest and its too late by then.