Increasing Training Frequency

I was wondering what everyone’s thought were on how quickly you could increase training frequency and prevent overtraining. I am specifically referring to running sessions leaving all other factors e.g weights the same. I want to add in another speed session in a couple of months and i’ve been doing 1 high intensity and 2 low intensity days for the past 2 months. I realise this depends on the individual but a general estimate would be appreciated for how long it would take to move from 3 running days to 4.

Are you recovering from injury, overtraining, other?

Thanks for the link I suppose I could use a strength endurance day as a bridge to 2 HI days. I’ve just got back into athletics after a few years break I used run around 4 times a week when I was 16 but now i’m 20, I want to steadily build my training frequency back and I was wondering how long everyone thought it might take to reach that level, just so I can have a rough plan for the year.

The more tempo the better, in my own personal experience.