Recently, a friend of mine at About.com and I were talking about our exercise schedules and ideas, and I mentioned my foot/toe numbness problem to her. Like the thorough resource that it is, About.com has an article on this exact issue: Why Does My Foot Go Numb When Running?
Earlier this month, I'd also stop around the 30-minute mark to give my toes a break. Now, I just push through my workout and worry about the tingling at the end, not wanting to break up the momentum.
More from About.com: Your feet swell when you run, so you should be wearing running shoes that are a half size to a full size bigger than your street shoe size. If you have a wide foot, you may need to get a running shoe that has an extra-wide toebox.
If you think your shoes are the right size, you can also try stopping to stretch when you start feeling the numbness. Sometimes tightness in our legs may lead us to run with improper form, which may put pressure on a nerve and lead to the numbness. So a quick stretch of any part that feels tight may help. When you stop to stretch, also try to move your foot around and massage it a little, just to get the blood flowing to the areas that feel numb. Running on your toes for 30 seconds or so can also help.
What kind of running or walking shoes do you wear?