It'll be challenging IMO. When you say time to train is now limited, how limited? I think you'd need to do another 25k, then at least one 50 (ideally two) and at least one 80k to be reasonably confident of finishing the 100k. You may be able to jump straight in and do the 100k with no problems, but it will always feel nicer to know you can do an 80 and only the last hour or two (depending on how hilly it is!) is new to you.
And is there much of a time limit for getting around the 100k? Can you switch to the 60k on the day? Usually I think you can, although you don't get any difference in entry fee back. You might possibly even be able to switch as late as when you reach the 100/60 course split.