You don't really have time to head anywhere in to town with a two hour layover. The arrivals process at HND has been very fast the last couple of times I've been there, roughly 35-40 minutes plane to train. There is a JAL recheck for domestic connections at Terminal 3 and then it's a short bus ride to Terminal 1.

There will likely be some JAL reps at the plane from SFO. Also quite a few of them in the Arrivals area in Terminal 3. If you think it's going to be close then talk to them and they'll advise you on your connection. JAL will rebook you to the next flight if the misconnection isn't caused by you such as Immigration/Customs issues or the inbound is running late. There's a lot of flights from HND to OKA all day. After all it's Japan's version of Hawaii!