The longest stages take between 4-6 hours for the riders on the bike at a fast controlled pace. You couldn't however, have runners running/jogging/slogging/walking 4 to 6 hours a day. I would say a 'long' running stage would most practically be 2 hours at the longest. This wouldn't necessarily be a fast pace, even when compared to an athlete's marathon pace. A distance that took the racers an hour to cover would allow for more tempo running day in and day out.
The 19k time trial this year was won by American Dave Zabriske in just a shade over 20 minutes. Lance was two seconds behind. This is an all-out effort for them, so I would just go on a relative time comparison and suggest a 4 mile running stage, completely flat.
When it comes to the mountains, you could take a page right out of last year's tour, and get a race from the bottom to the summit of le Alp d'Huez, which I think is ten miles long.