Quadricep Tendon injury - How to recover.

Hi all.
About ten days ago, I noticed that my left quad tendon was sore, but I kept running. Since I had to do 21k on saturday, I took friday off to give it a bit of rest; it help a bit. But by km 5, my tendon was acting up again, so decided to stop. I ran a 10k on sunday, felt great and did good time, but regretted it afterward, my soreness became more painful. I started limping, could not walk up & down the stairs, could not walk properly (had a limp).
On monday I went to the doctor, who told me I had quadricep tendonitis. Started therapy: heat, ultrasound & magnetic therapy (not sure of the last one, but it does not hurt). Went through this monday-friday. By friday, I could walk up & down the stairs, still sore but doable. Walking was fine. The doctor (a sports medicine doctor), told me to run but only 20k. I tried, but by km 3, I could sense that healing was not properly done and it hurt to run. I know I could run through the pain but it meant getting worse, so I stopped and walked about 14-15k.
During that day (saturday) I applied ice several times. On sunday a couple of time and did about 20 minutes of pool running.

Well, my question is how to recover from this !
My marathon is in less than 4 weeks. My doctor think I can still make it, if there is recovery. He suggested doing bike meanwhile, and wait until wednesday before trying to run again.

Should I apply heat? Should I give it massage? How? Should I do pool running?

On sunday I have another half marathon and if I can't run it I'm seriously considering ditching the marathon itself.

Any ideas will be greatly appreciated.