Put simply... distance, distance, distance.
When you've got lots of strength and endurance in you, then start to add the occasional bit of speed work.
That's what you should concentrate on... but of course, you don't want to bore yourself to death... so if you want to do a bit of speed work... and you are confident that you've built some strength back up, then why not do a bit. Make sure you warm up really very very well... and cool down too. e.g. 10-15 minutes gentle running either side of your faster stuff.
Glad you're back into it... good luck