I am 68 and use a treadmill for 2 hours per day, is that healthy?

That is incredible!!: I don't hear that too often these days--doing 2 hours worth of exercise/day at age 68 is awesome. You actually answered your own question. If you are not healthy, you won't be able to do that for 2 hours. I am beaten by nearly 30 yrs and i don't think i can last 2 hrs on the treadmill--hope i can do 1/2 of that at 68. Keep up the good work--i can see u beating 100 yr mark. Good luck.

Qualified yes: 30 min of moderate exertion daily is all that is necessary to achieve benefit. If you are doing more than just walking on the treadmill, then you run the risk of overuse injury with no net gain. Clearance from your doctor is always advised prior to starting , or significantly increasing any regimen.