So I do a 4 mile run about every other day sometimes every day. I start off jogging and build up, by the time I finish im sprinting. I also cycle up hill. I can handle this run easily. I don't get aching legs or out of breath etc. It's not that hard for me. But last week I went to run and I just couldn't finish. I got to 3 miles and my thighs were agony. My stamina also wasn't what it normally was. I just couldn't go on. So I rested and hit cycling today and again it was so much harder than normal. My legs were feeling pain so much more than normal. I don't understand what's happened. I was pushing along nicely working up a great fitness level and it has just suddenly gone. What could be causing this? Thanks.
Try actually pushing yourself then. Could have been lack of rest, nutrition, you're not with it mentally, many things. Unless this is happening consistently I would put it down to an off day.
There could be many things going on but given what little info you've given here is my suggestion. Drop back to slow long distance runs between 45-60 minutes for about a week or so, 3-5 x week. Keep your heart rate between 130-150bpm. If you choose to cycle your heart rate should be about 10 beats lower than running(due to being seated). This pace may seem painfully slow but do it. If you do not have a heart rate monitor, you should be able to have a conversation with little effort when you hr is in the 140s. I think the worst thing to do is absolutely nothing. This type of training helps with recovery. Good luck.
It's likely either poor nutrition or you're overtraining. Why you'd be doing this same run almost everyday beats me, I don't see the point in it.