How long until I notice the effects of running on my legs and abs?

ok so i started running couple weeks back. so far all I've done is a 5 mile to start followed by a 4 and 2 lots of 7 miles. these are spread over a 2 week period.I'm gonna up it this week a 7, a 5 and a 3 each week. I'm not particularly keen on doing more than this as I'm a bit apprehensive about potential damage to my knees. I'm already slim and want to try to maintain my weight so have added protein shakes to my diet for extra calories. I was wondering how long its likely to be before I see a visible improvement in my leg muscles. I am slim but I build muscle quite easily. I already use weights on my uper body but as I don't like doing squats and lunges etc I thought id try running. I'm amazed at how easy I found my 7 mile yesterday to when I ran it over a week ago so I have noticed a change already with regards to my endurance.