What, long As like Duddon Valley, Ennerdale, Wasdale, Borrowdale, Three Shires and Langdale?
If you have done the 3 peaks then the Duddon wouldn't be too bad, as it is shorter, but has much more climbing, is a lot rougher, and there are no flags to show you the way round. My times for the Duddon and 3 peaks are similar, if that is any guide.
Ennerdale and Wasdale are complete bas%@£ds and a 'kin long way, you can add an hour and a half to two hours on to your 3 Peaks time on a bad day and an hour on a good one!
Borrowdale is similar in length and time to the Duddon, though it is rougher and has less good running - the second half of the Duddon is very runnable if you are up to it.
Three Shires is one of the best races in the calendar, but is not really in the same league as the others as it is very short by comparison, though has a lot of climbing.
Langdale is also a fantastic race - longer than Three Shires, but considerably shorter than Duddon or Borrowdale, such that you can finish it running rather than staggering and don't have to sit down at the finish for half an hour before being able to contemplate moving anywhere (or is that just me?)
Basically, if you can get round the 3 Peaks you will probably get round the Duddon ok, so long as you have some experience of hill navigation, and once you have done that you can do the rest, though doing Ennerdale the week after the Duddon is a challenge in itself, though most of the Ennerdale field will be doing just that, but they are mad!