Hi Riv. Welcome to the forum.
The first thing you're going to find with horses is that they cost one way or another. In your situation, given your experience and circumstances, the best thing really is to buy some extra hay (you're going to need it for winter anyways) and leave Lily in her pasture. It's the safest way to do things in the long run - I can assure you a vet bill for a rope burn or some other disaster is going to cost you a lot more than the same amount of hay she needs to consume while she's in the small pasture. If you want, when you have time you can bring her out and hand graze her (ie she's in her halter and you're on the other end of the lead rope); that will save a bit on hay use, give her a chance at green grass and give you two a chance to bond.