Right, I have ready everyone's replies and I am very grateful for all your help!
I guess I will need to pay greater attention to my eating and drinking habits. I did not think at this stage I would need to! But it's clear that I need to start. Perhaps some form of sports drink is the way to go.
Here's a question though, I'm very annoyed that I haven't lost any weight since I started cycling (in fact, I'm gaining). So if I'm not losing weight, but I'm not fuelling enough, what do I do? Surely any extra calories I take in is going to result in a greater weight gain?
I have been thinking recently I need to cut down my calories to enable me to lose weight. How do I balance this with fuelling my rides?
I think the side stitch could be a result of not warming up. Now I think about it this problem has only occurred after I changed my route, which has a large hill right at the start. I guess I could be exerting myself too much too soon.
Another technical question, my bike feels very inefficient. I would like the change the tyres for skinner, slicker ones. This was always my plan. I currently have 700c x 40 tyres on there and they're really knobbly. They feel like hard work. Can anyone explain to me what size I could go down to (so 700c x 28 or whatever) and if slicks would be recommended on a hybrid? Every day I regret not getting a road bikelol I am constantly being overtaken by them!