Posted: 09/03/2015 at 19:42
It can vary person to person and I am certainly no expert but I would say go on feel which is what I do. I tend not to have too structured a plan due to working shifts which have no routine on a week to week basis so sometimes I do have to shift things about. I also know what you mean about being keen to train every day.. I am one of those who hates a rest day, unless I am actually knackered.
Some training plans (IronMan) for example don't have definite rest days written in as there is so much to train for but they might have easy days where there is the option for a "recovery swim" or a rest.
I'd say if you are really keen on training most days make sure you at least have recovery days where maybe as above you just have an easy swim or similar. That way your body gets a rest from the heavy stuff and your legs can recover but you still feel you have done something. Otherwise you will eventually knacker yourself.
In general though if you are marathon training I'd say 1 or 2 rest days... and some other days will be easier as well...