Just don't attempt to walk from the main gate unless it's first thing in the morning! Our taxi dropped us off there, just off the main road and we paid our $5-6 entry fee and set off on the walk. We knew it was a few kilometres but we weren't prepared for the lack of shade and the incline to the top which I believe is around 5 kms in total. It was very hard going in the afternoon heat! The visitors centre / museum which is definitely worth checking out is about 1 km from the main gate and then it's another 4 kms up from there. I would definitely recommend that you drive or catch a taxi if possible.
Once you reach the top you will see a lot of smoke but be patient for the wind to clear and you will get a better view into the crater! It's pretty awesome to be able to peer into an active volcano, but don't expect to see lava or too far down.
The path up to the cross is unfortunately still closed off so you can't get any higher or see much more than the views from the side.
Finish the trip by rehydrating with a fresh coconut for a $1 from the local ladies who have a little stall at the top.