Just get on a bike and ride! I am getting back into this and I probably would have asked the same question since im doing it for weight loss as well (fun too). The reason I didnt is it will be like a crutch...or an excuse to give up. So and so said I only need to go for 30 minutes so I can quit now when in reality you arent even warmed up and can go for another hour but you lack motivation to do so.

So just get out and ride...use your body as your gage. If you are out of shape, maybe 15 miutes is all you can do at first...so what? Bump that up 5 minutes a week and before you know it you will be cranking away. Anybody who gives you crap for riding for 15 minutes or even 10 if thats all you can do is an idiot...do what you can but push yourself while doing it.

Like I tell myself about everything I do...if it wasnt hard I wouldnt bother doing it.