Good luck. Not to be a downer, but as a non hard-core cyclist without dedicated gear already, I suspect that 13 miles on a mountain bike on busy roads in fall/winter on the east coast will be too much to handle for a new bike commuter. Not impossible, but I guarantee it will be difficult, unless you really, really enjoy being on your bike.

Despite what folks on this forum say about needing minimalist solutions, for your kind of described commute, I anticipate that you will actually NEED most of the equipment you hear folks talking about. Like:

- Raingear
- Lights - serious front and quality rear
- Fenders
- Rear rack+panniers or a bike-friendly backpack
- Hardcore reflective gear

None of these are optional if you're riding for an hour+ in darkness, rain and with significant car traffic the whole way. All of these are optional on a road bike in summer conditions with a <5 mile commute, but in the tough conditions, they are not. And on the east coast, it's more likely than not what I describe.

And on a mtn bike, 13 miles in traffic will be a chore if you're riding in the dark+rain, which will be fairly soon. I do it (18 miles one-way but with bike-friendly roads), and so do many on these forums, but I'd consider myself "hard-core" in those conditions, and honestly, in dark+rain, I find it no fun to be out there. Plus, I'm on a road bike, which is a good 20% faster than the mtn bike, easily.

Definitely take it easy at first - once a week is plenty with 13miles on a mtn bike, and do it on days with great conditions and when you're feeling great. If you force yourself too much early on, I guarantee you'll stop even considering it.