Get yourself a GPS and go for it! I rode from Koh Samui to Singapore with my wife in a pretty spontaneous way, and ~1500km in 3 days was A LOT of fun, so I don't think it will be a problem for you.
If you go from KL to Krabi, I recommend you stay overnight at Tasoh lake. There's a hotel right at the lake, the place is just great and not so expensive (~$40 per night). Here are some pictures (sorry, I just can't figure out how to attach them to this post directly).
PS - we also made a trip from Koh Samui to Golden Triangle and back during NY holidays, it was also a great time. You can see some pictures here.
