How much is too much?

strader

Do you feel fatigue the day after? Generally speaking, I think you should be okay as long as you listen to your body and rest when you need to. Never ride with any soreness in the joints. If you have sore muscles an easy low intensity ride can help you feel beter and still burn some calories. If you feel generally tired it might be better to just take the day off. The mileage you are doing is a lot less than what many bike commuters do every day. I don't see much risk for overtraining, but you may want to mix things up more instead of doing the same 4.5 mile loop every day to prevent boredom.
Many people take pre-planned rest days and rest weeks, because they read they should in a book or on the internet. I used to follow this approach, taking a day off after any high intensity or long duration (>1hr) ride. As a result I reached a plataeu pretty quick, and stopped improving. Now I take rest days when I'm wiped out, at the end of an intense multi-day training block, or when I need to rest-up for a race. My race performance has improved leaps and bounds since adopting the new approach. As your body adjusts to the higher training loads you can ride faster, farther, and recover sooner.