Started running 5 weeks ago after 1.5y hiatus. 4x/week at 3-6km each. no prior issues now severe pain under cap. can chondromalacia. develop this fast?

Prob. not: More likely diagnosis considering your history is patellar tendinitis.Try taking Ibuprofen or similar NSAID for a week , ice massage to 'bottom' of kneecap( patellar tendon origin), and cross- training for 2 weeks. Slowly start to jog/ run after this time starting @ 1.5 k and adding .5-1k per wk.Until pain free and running w/o pain. Best of luck!( see ors for definitive dx.)...