Best bet is to get up to Krabi Town from Lanta either by bus or boat.
If you go to Krabi town bust station there are buses up to Kuraburi. They take around 5 hours and cost a less than a couple of hundred baht. Most of the buses are in the morning. I seem to remember there is one around 11.50. It is unlikely you could get to KPT the same day if you take a later bus as it can be a mission to reach.
You can also get buses from Phuket town and there are plenty heading north.
Hope this helps.
If you catch one of the early minivans off Lanta via the vehicle punts to Krabi (they pass by Krabi bus station) you will make it easily in one day.
There are fewer direct Krabi to Kuraburi buses so if you find yourself on the more common Krabi to Phuket bus, don't go way down to Phuket bus station 40km down the island only to come back up - get the bus to put you out at the Checkpoint at the north of the island. Cross to the other side of the road and wave down one of the buses crawling thru here. What you ideally want is a Ranong or Chumpon bus (destinations are written on the front in Thai and English and they usually stop anyway and ask where you are going) which go thru Kuraburi. Buses to Takua Pa or Surathani/ Khao Sok will work - get off at Takua Pa bus station where there are local buses to Kuraburi and where later buses going to destinations mentioned above will call in.
Note that if your final destination is the Surins, the National Park has closed down for the wet season by now. But maybe you are doing some early planning for next season.