Total lack of fitness

[Biciclista]

since you brought up clipless pedals, I have to ask you how you had them set up. There is no way that my feet could remain in the pedals if i was falling, i have mine set so loose so that I can pop out of them (SPD's) however and whenever. I have fallen (and almost fallen) several times and I can assure you, I do NOT stay clipped in. It's an instinctive reaction.

[Catrin]

I've not noticed a significant speed difference for me when I am clipless and when I am not. I ride clipless because I've less knee issues with them rather than without - I've been toying with the idea of heading back to my BMX pedals but I probably won't.

[Dogmama]

You don't say how old you are or what your experience is with riding & fitness in general. So - if you're young and you were previously very fit, you can probably ramp it up fairly quickly. If your are older and/or you were not in great shape when you stopped riding (great shape = 40-50 mile rides were doable) then you do need to go slowly. The rule of thumb is to increase by 10% each week. So, if you ride around 50 miles/week normally, next week you'd ride 55 miles, etc.

Spin classes will definitely help.