Returning after injury

I was diagnosed with Knee tendonitis in January by a PT, before this i had built up quite a good running base, done a few 10k's too.

After 6 weeks off I have started to run again but run/walk to get back into it but i am finding it so hard, not on my body but my breathing, i did use exercise bike while not running and walked, i ran/walked last night after a week off because of the snow and it felt so hard, will i ever get back to where i was?

Michele