The very best medicine for bad feet is exercise, so don't you worry about having to chase her - in fact, chase her about anyway, it gets the circulation going properly into the toes.
If you want to do the IM injection, the easiest place is the back of the back leg - the fleshy buttock / ham / gigot muscle. Don't do this in a fat lamb, it may leave an abcess and the leg would be condemned. But in a keeping ewe you'd be okay doing it there.
Another good spot is inside the thigh, but the same applies about not doing this with a fat lamb or sheep heading for slaughter anytime soon. For this one, tip her onto her backside, press the fleshy thigh to find the leg bone, then stick the needle in well behind the bone.
The spot most farmers and vets use, and which you can use in fat lambs too, is the top of the hip. There's a fleshy triangle between the spine and the point of the hip bone; I could show you but find I can't easily describe it, hopefully someone more articulate will be along shortly...
I use a 1 1/4" needle for I/M in a sheep. 1 1/4 x 16, I think it is. Someone posted a 'which needle for which jag' list once, couldn't find the one I wanted but
this one from Fleecewife is useful.