@bobnaquin Hi Bob, and welcome to Connect.
I have had two TKRs (Total Knee Replacement) and have fared quite well after both of them, but I have not done anything as strenuous as sprint running. I think that is something that you really need to discuss with your orthopedic surgeon. I exercise a lot, but I do not run, but I didn't run prior to my knee replacements either.

Regarding when to start PT, they generally start it while you are still in the hospital. I presume you are on Medicare since you are 70. Medicare pays for in-home PT as long as you are "homebound" and that can start immediately. When you are no longer homebound you start outpatient PT. My surgeon recommends "gentle" PT and doesn't even think that formal PT is necessary if the patient knows the right things to do to rehab the knee. The in-home physical therapist had worked with other patients of this surgeon and knows his high success rate so he abided by the recommendations. My outpatient physical therapist was frankly stumped about what he could do for therapy so those sessions were a bit of a waste.

My surgeon is cautious about the strong impact to the knees so that may be a factor with your surgeon also. I do water jogging and use my recumbent bike, a treadmill, and elliptical. I will be curious to hear what your surgeon has to say about this. I think it may depend on the model knee you have, and also the condition you were in prior to the TKR. I hope you will be able to resume the activities that are important to you.
JK