Nutrition, rest - or just my own fault...

Just come back from a personal training session in the gym (on a separate note its great, has done wonders for my core strength muscular co-ordination etc). Really faded about 45mins in (though was working hard and anaerobically for a lot of that period).

Could it be:-
- Ran and swam yesterday and didn't eat enough carbs afterwards? Breakfast (cereal) after the run and a Gatorade after the swim then dinner about an hour or so later
- Just went too hard?
- Still need to work on my endurance?

Think it might be a combo of i) and ii), if that's the case I just refuelled really well after this session (fruit juice, banana, power bar) will that be enough to restore glycogen to train tomorrow or am I going to have to take a rest day?

Would appreciate thoughts, I know the best thing is probably just to go for the run in the morning and see how after feel after the first km but would like to be a bit more scientific!

Thanks!!!