Running more

I have been training for a couple of months about 3 miles at a time about 4 times a week. i did a 6 mile run and injured my knee a month ago but got it sorted with a physio, the problem im having is i cant seem to run more than 5 miles without my legs hurting or feeling fatigued. my heart rate and lungs feel fine its just my legs. should i just keep pushing and hopefully pass through this ir is it a deeper problem?? thanx for ur help,.

Have you been properly fitted with a good pair of running shoes? A pro can match a shoe to your inate biomechanics. If your shoes are worn out, they may not be absorbing shock or supporting your foot properly.