For me, the rhythm part of a tune is the easiest to get. If i can at least keep good timing and chording, i can play rhythm---i don't really have to take a lead. I start out with my met at a slow tempo, 40-50 BPM. If it's a 4/4 song, i'll strum with the tick of my met, bass/strum/bass/strum for a 4/4 tune. I don't do anything fancy while learning the tune, no alternating bass line, etc. I'll do the alternating bass when i can get the chording clean and on time. For a 3/4 song, i use bass/strum/strum. You should only play as much as a space of time allows. This is where a met is an excellent tool.
Our tunes are written in 2 part, lead and rhythm, as we would play them with others. I have found that when you learn a tune good enough, you can improvise with a blending of the lead and rhythm parts to make it sound "musical". I did this experimenting on Amazing Grace. I played a combo of lead and chord in the measure. I would substitute a lead note with a chord strum. I even did a version in 4/4 time, playing 3 lead notes with a chord strum. Again, depending on the time signature, let the time signature be your guide, don't play where time doesn't allow.