You could also let the valets park your car at the Poly, then monorail or boat to MK, and monorail or boat back to Poly at the end of the day. If you're staying off site, the cost of valet parking + tip is within a few $ of the regular theme park daily parking fee (free if you have a disabled parking permit.)
I don't know this firsthand, but others have said here that it's OK to leave your car at a resort all day if you are using valet parking.
That way you'll be right at the door to the Poly -- no walking at all. Taking the boat to MK & back would avoid having to descend & climb the ramps at the MK monorail. There is a step down into the boat, but it's about the same as stepping into the boats at POTC or LWTL @ Epcot.