First of all, the only way the game will end is if a bomb explodes (or if you make a black pearl cluster or starflower of pearls). The bomb is the real problem though. So, if a bomb comes on the screen, you must drop everything else you are trying for and make sure to clear that first.
In terms of obtaining points, you should keep making starflowers. You make a starflower by surrounding one piece with six other pieces of a different colour (it is explained in the game but just thought I would add it here). Once you have two pieces, you can then use them to make other starflower pieces a lot more quickly and easily. Just continue making the pieces and when you have enough, you can make them into a cluster or a black pearl. Try to keep at least two starflower pieces on the board at one time to help you better negotiate the board. Also, if you have a special star piece as one of the 6 when you create a starflower you get extra points for that.
As I mentioned before though, just clear those bombs out straight away as these are what will stop you from reaching your 150,000 point target.To note (as mentioned by SmokingBmbr465): you can save your game progress for later by pausing>return to title screen Next time you select Marathon mode, it will prompt you to continue your game