How long for legs to get used to fixed gear 15 mile daily ride Due to a car accident and the extended time insurance has taken to get my car handled, I pulled my fixie out of the garage and started riding it to my university mon-fri; it's about a 15 mile round trip, no hills. I have been doin this ride for 3 weeks now and my legs are still significantly sore from the ride every day. When I am riding, there are no issues with my legs or stamina or anything, but lets say I am standing and I keep my leg straight and try to lift it; at about 45 degrees the top part of my leg where it meets the hip/pelvis there is pain and I cant really lift it any higher. Is there a certain point where after having done this ride 5 days a week for a given number of weeks that I will not have this issue anymore? If anyone knows any stretches for cycling too that they would like to suggest, that would be appreciated too.

For profiling on this issue I am 5'5", 120lbs, and have been significantly inactive for the past year as a university student (literally spend the large majority of my time sitting in a chair).
First off, I assume when you say "fixie" it is a pure fixed-gear bike? It's not a flip-flop (where the wheel can accommodate a fixed cog on one side and a freewheel on the other)?

If it is a flip-flop, you'll find it easier to ride single speed rather than fixed.

Second, I agree with much of what @Arne has said in your comments. From these you say you're riding 45x15 - I think one of the beauties of fixies is that they're easy to work on, and you can easily buy cogs with more than 15 teeth. Similarly, but a bit more expensive, you can easily buy a chainring with fewer than 45 teeth. Either of these (or both of them) will make your ride easier (and therefore ease on your muscles). 

A site I like is Velosolo, not so much because they have great prices or anything (although I have bought from them and it was a good experience), but they have a good selection of single-speed/fixed-gear parts, so you can evaluate what your options are. 

But make sure you keep your current parts as you'll likely want to put them back on the bike in a month or so.