4-5 miles is a little much to just start out with, if you haven't commuted by bicycle before. My commute each way starting out about a year and a half ago was about 3.5-4 miles. I was in relatively good shape and it was still difficult at first. I'm sure you'll have some story of climbing Mt. Everest or bicycling naked through the Sahara desert that will prove you can handle it, though.
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Some things to consider when you are weighing the pros and cons:
1) Clothing - if you are the type who likes to dress nicely/in a skirt or dress, it is very difficult to do so when bicycling, particularly for a 5 mile commute. Long dresses or skirts get caught in spokes/wheels or get dirtied by the chain, and short or swishy ones expose....a lot.

You will also get dirty/sweaty a lot of days, particularly if it is windy, hot or rainy (basically anything but 70F with low humidity and light breeze).
Way to circumvent this: Leave a lot earlier, dress bike-appropriately and bring other clothes with you (means more stuff to lug around)
2) Major roadways you'll have to be on - Obviously freeways are just plain out, but if your route goes over somewhat-major roadways that are busy during rush hours in your town it can be difficult to navigate these. Riding on the sidewalk is
completely inappropriate for using your bicycle to commute. It is a vehicle, treat it like one and ride on the road (and consequently, follow the rules of the road) if you are going to commute by bike. Riding on the sidewalk is rude to pedestrians and difficult for motorists on the road to react to when you are crossing. In many places it is illegal, as well.
Ways to circumvent this: Find alternate routes that include bike paths or trails, or use less traveled roads.
3) Laws regarding bicycling in your town - kind of goes hand in hand with #2, but be aware of any local laws or ordinances regarding bicycles specifically where you live. Know the helmet laws (here it is under 18 must wear one), know laws regarding reflectors or bike lights at night, know what the penalties and punishments are for breaking these laws. Here in Davis your drivers license can actually be suspended for certain bicycle infractions, Know your rights on the road.
4) Cargo - you said you don't know how much you'll have to carry with you, but obviously what you can haul around on a bike is dependent on whatever you use to carry it (personally I have an amazing, waterproof, on-the-fly adjustable Chrome messenger bag), and if you put a basket on your bicycle or not. If you'll have some space on campus where you can generally leave books or heavy/awkward things that you might need while you're on campus this might not be an issue. If you need to stop off at a store to buy a couple things, you have to limit yourself to what fits in your bag or basket. This is sometimes a
good thing for me.
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5) Maintenance/random gadgets you need - sounds like you got a good start on this. In my under-seat bag I carry a bike multi-tool, spare batteries for my light, and a spare key for my U-lock. The U-lock mounts on to the frame while I'm riding, and I have a pressure gauge/pump mounted to the frame as well. When I park my bike I lock it to the rack, then remove my under-seat bag and my light. Those things
will get stolen. It's also necessary to keep your chain lubed and everything tightened up, check your tires and brake pads routinely (also know where there are tire pumps in your town). You'll also need a good set of fenders if it ever rains where you are.
Those are some randoms that people may or may not think of when they consider switching to bike commuting. It's really good for your health and for the environment, if you think you are up for it, so I totally encourage it if you can!
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