“I know I'm supposed to build up slowly, but I'm so unfit right now that I'll just hammer for a few months, get some basic fitness and then start following a plan.”
Bad, bad idea. Especially since--you corner-cutting basstid--you'll just get too lazy to do the whole plan thing and just ride hard all the time.
So stop it right now. Unless of course you want to waste two or three years riding hard, getting a little better, still riding hard, getting a cold, recovering (barely), riding hard, getting another cold with fever, recovering for longer this time, riding hard, JAYUS ANOTHER COLD??? Not to mention the untraceable general malaise that seems to follow you around like a dark cloud. And most important, that you stop getting better and just fluctuate between being average and recuperating.
Thinking back now, I can't believe what an idiot I was. I so badly wanted to be fast on the bike that I'd just jump on and after a token 20-second warm-up, I'd hit redline and stay near it for the whole workout (as much as I was able). I'd rest a day, then I'd do it again. Another day's rest, and I'd do it again. I'd feel like shit and wonder why. After all, I'd rested in between workouts right?
If you ride like this and finally get on to a training plan, the first thing that'll hit you is how easy the plan seems. A hard day or couple of hard days is always followed by rest or easy days. Workouts with periodic elements (intervals, sprints, hill repeats) always end just as you feel you could do one or two more. It's very rare, if ever, that a workout is designed to “kill” you—staggering home on dead legs is almost never a goal.
You've heard it from the experts over and over. Now hear it from your friendly local idiot. If you want to get fitter, faster and ride further, don't get on the bike and ride like a demon with diarrhoea. As they keep telling you, you don't get fitter on the bike. That's when your body is just coping with all the screaming demands you're making. You get fitter when asleep in bed at night, when the elves come out and start repairing tissue and making it stronger, drilling out more capillaries, rewriting the fat burning code to make it more efficient, bracing ligaments and the many other little changes and improvements that are what we collectively call “fitness”.
The other aspect of rest is that you approach the next workout with enough energy to make it count. Without it, you gradually do less work (the physics one) during a workout, and therefore gain less, even though you feel as if you're driving yourself very hard. Never forget that just because you're panting and tired, doesn't mean you're doing a quality workout. When you're ill, just getting up and walking to the bathroom is tiring, but that doesn't mean it makes you fitter.
Whenever you do a workout, you should do it with the rest of the week in mind. If you know you must get five quality workouts in a week, you're much less likely to murder yourself on Monday. Also, play an even longer game. Remember that once you start exercising, you'll feel so wonderful that you'll never want to stop. What sort of schedule can you (and will you) sustain for the rest of your life? One where you push yourself to exhaustion five days a week? Not bliddy likely.