you're at the point now where you have some time to burn and all you need to do is fly. What you should do is fly until you have about 230 to 235 hours in the 172 and then get with an instructor for the rest. That 20 hours will be spent basically in checkride prep and getting you comfortable with the airplane for the checkride. That 15 hours can cover all the requirements easily. Its not a very hard ride either, basically a private pilot checkride with a few more maneuvers. I did my commercial my sophomore year of college and the best part of it was building time flyin to random places with my buddies and taking girls flyin. Start on your cfi as soon as you get your commercial rating also because a commercial without a cfi is pretty worthless and all that information will still be fresh. You don't have to do extra instrument training, that part is included since you technically don't need to be ifr to have a comm rating so you can exclude that part. Just make sure your instructor puts you under the hood for a little bit because you'll probably get some unusual attitudes or timed turns or something during the test.

P.S. The day and night 2 hour cross countrys are supposed to be dual. When the regs specify "training" they are implying dual with an instructor. Do this stuff in conjunction with getting checked out in the airplane. Also get the solo stuff out of the way now and make sure to specify that it was a "300 nm x country for comm rating solo" or something like that so it is easy for the examiner to find when they go through your log book. They love it when you have it all organized.