You become less fit.
That was the short answer. There is a much better long answer here:
This is your last day of training. You have decided to take no regular vigorous exercise.
No discernible decrease in fitness. If you have been training hard up to day 0 you can expect your fitness to be higher than day 0.
Day 7 (Week 1)
Fitness begins to drop due to decrease in blood volume and muscles becoming less efficient at using glucose and at coping with lactate build up.
Things are starting to slip. Your maximal oxygen uptake (VO2 max, the prime measure of your aerobic fitness) has dropped by up to 20%, because the muscle mass in you heart has dropped by 20% and the number of capillaries in your muscles has declined, resulting in a reduction in oxygen uptake to the muscles.
You are definitely on the slide. Your muscle structure is reverting to that of a normal, non-exercising person. Your muscles are less able to burn fat for energy and are no longer able to sustain high intensity efforts.
Your heart now 25% less muscle and your muscles ability to produce energy from oxygen has dropped by up to 45%. You now start to produce stress hormones when exercising which makes it mentally difficult and increases recovery time.
You are now unfit and have probably put on weight. Even if you still weigh the same you will be fatter, muscles mass having been lost. But the good news things have mostly stabailised, although your ability to utilise oxygen during exercise is still declining.
Stop training and your fitness begins to decline quite soon after, continuing to fall away the longer you stay off the bike, until after six months of inactivity, you’ll be a long way behind in the fitness stakes.
What Can I do?
Interval sessions consisting of just 8×30-second efforts, two or three times a week – just 12 minutes a week – can be very effective at preserving aerobic fitness.