This is my second ten day MC, I've done a dozen or more three to five days fasts in the past including MC, juice, and water only. Master-Cleanse is the easiest to do, and I've never gotten weak. Even with the healthiest eating habits you are exposed to enviromental toxins, plus a strong probability of residue from days of not eating so healthy (unless you're an exceptional person, there have been some unhealthy foods in your past ;-) )
To go back to eating, take it slow and easy, especially if this is your first time. The general rule is on day eleven drink plenty of fresh squeezed orange juice. I think its best to dilute it the first few times you drink it. The next day continue drinking the orange juice although that evening you can have some veggie broth. I make mine by chopping one unpeeled potato, one carrot, a couple of stalks of celery, some onion garlic and parsley (to your taste) into a pan of pure water. Gently simmer, do not boil, until the veggies are tender. I add some Mrs. Dash, a pinch of Sea Salt , and a tiny pinch of cayenne for seasoning. Drink this broth, you may need to dilute it. On the thirteenth day reintroduce solid foods. A little thin oatmeal is good for breakfast, or some mashed fruit.
Try the vegetables from the broth for lunch. The Master Cleanse book suggests raw vegetable salad for dinner, but I can't handle that for a couple of days. I think lightly steamed veggies and brown rice is a better choice. Its fairly bland, filling and won't mess up your stomach. Eat lightly, but fairly often the first few days and everything should be fine.
BTW-I usually eat like this after a liver cleanse too, except I drink apple juice.