When and how to re-introduce training after infection/illness/man flu!

Been ill now for 3 weeks getting better, lost a stone in weight due to lack of appetite, coughing all the time (phlegm) , difficult to get any sleep, temperature, uti etc. So now 3 weeks in am still feeling really weak but now sleeping a lot. My question is how do I get back to how I was 4 weeks ago in terms of fitness? My goal is to do the 312 route in Mallorca in the beginning of April and certainly going backwards instead of forwards at the minute. Cheers Scott