I am an 80 kg guy, height 5'6", and I have been jogging for like 5-6 months now. Initially, I used to jog in a park, 1-1.5 km a day. Then I gained a lot of weight and didn't have time for jogging, but I have resumed jogging. My stamina increased, and I started running 4 km — this was on hills, with little slope up and down. My quadriceps hurt like hell initially, but the pain went when I stretched a little. Then I had a break for a month or so, and again I started running. I tried to increase my pace of running to 10-10.5 kph, but this decreased my endurance and I could only run like 2.25 km in 2 laps. Also, now my legs hurt. The first 1 km is easy, then I take a break, and the next 1 km I feel pain. I also feel the pain in day-to-day routine as well. I haven't stopped running, but I only run like 3-4 days a week now.
I want to know what actually is causing this pain, and what I should do now; I want to be fit and keep running, and increase my pace and stamina as well. Thanks.