How do i get my fitness back after a long break?

Those of you who saw my (re-)introduction post will know that I took some time off running, about a year. I started up again last night and I noticed that my general fitness has (obviously) dropped quite dramatically.

I have mild asthma that previously had never bothered me whilst running, but last night I was wheezing (as opposed to being out of breath), which I'm not putting down to my asthma getting worse but rather my fitness level being lower.

I struggled with my breathing and had a lot of phlegm on my chest/bunging up my throat.

Basically, what would you guys recommend I do to get my fitness back up? Sprints? Just carry on running, what?

I run every other day (well, I'm starting to again, as I had previously been doing) and I do the same loop every other night. Is it worth changing up my loop? Will this be beneficial do you think?

I do roughly 4 miles per run, every other night, about 3-4 times a week (depending on how busy i am on weekends)

Any/all help and recommendations are appreaciated.

Cheers!
-sc00t