Saddle or, as noted above, clipless pedals would be the only thing I'd look at till something breaks or wears out. Clothing is also a good place to spend a few bucks. Padded shorts, jerseys, gloves, etc. will make the long rides more comfortable.
If you really have some extra money, start a "new bike fund" then save up till you get enough miles under your belt to know what is really important to you in a bike then start shopping for a new one.