Strained hamstring tendons behind knee

Has anyone had this before? How long did it take to heal?

It started as a niggle behind my knee about 3 weeks ago, and got a whole lot worse after an 8 mile run about 10 days ago. I have rested and iced it since, am seeing a sports physio who told me strained tendons are the cause of my pain, I have done all the stretches she gave me etc and was given the go ahead to try a slow 3 miler yesterday as I was pain free. Well, I was less than a mile in when the pain started up again so I limped home, back to square one and back to the physio tomorrow!

Getting panicky now as I only have 6 weeks to go before my first half and my training was going so well too!

The actual tendons are not painful to the touch and there is no obvious swelling, so could it be something else other than tendons then?

Any advice gratefully received