[Closed] Getting back to fitness

Accidentally posted this in the wrong forum originally.

I've recently had a bad cold/cough that the doc said appears to have spread to my lungs. I know not to rush back too soon, but am eager to get back on the bike.

Therefore; when I am feeling good again, what should I start doing first? I will be using a road bike.

Can I build up a good level of fitness doing around 18-20 mile rides after work 3-4 times a week, and then mountain biking at the weekends? During the week I would probably focus on intensity, rather than endurance, basically because I don't have the time