Fuelling/nutrition is hard to advise (look at the water debate) as we all tend to have slightly different requirements, however to eat something is generally better than nothing. Your body stores energy as either Glycogen ( a glucose store) in Muscles and Liver plus fat. When we excercise we burn a mix of Glycogen and fat, in simple terms at low efforts the ratio tends towards more fat burning and at higher effeorts more glycogen is burned. Thing is the bodies glycogen stores are relatively low and need frequent topping-up compared to the average persons fat stores. Chances are you started-out with pretty low Glycogen stores and a slice of toast wasn't sufficient.
I reckon at a medium/fast pace 16-18mph average I'm good for 40 miles or so on water only and a couple weetabix for brekkie, but without more food I'd start to deteriorate quite quickly as my glycogen stores deplete. waterwise I tend to get through 500ml/50 miles. For a 20 mile ride, water alone is fine for me and nothing else if I've had a 'normal' diet..
Overheating is an issue, especially at this time of year. Layers are the answer that you can quickly remove to adjust to the conditions. I really suffer with overheating so it pays to be flexible and remove layers as you warm. Remember though to put them back as soon as you stop as it'e very easy to get chilled at this time of year. Just been out for about 25 miles around Oxford with a thermal base layer and a winter Jersey ... I boiled! Then our group had a P*ncture and standing around was very cold especially with sweaty undergarments. Better to dress to be warm but shed layers as you go, alternatively, slow down if you're overheating.
With practice al these things become second nature, you'll know what you need to eat and when, you'll learn what to wear for what conditions. Don't beat yourself up, you're doing fine, winter's a hard time to be starting-out, but come the spring it will be so worth it. 60 miler for the Oxford Classic in May?
Cheers FF.