This was maybe my biggest question when I was starting out. I obsessively looked at people’s posted stats to try to figure this out.
You have to understand that it’s a scale, and the low end of that scale is “you might not gain at all” and “you might gain length, but little to no girth.” Some people seem to have very tough tunicas that really don’t want to stretch and grow. Girth, especially, is harder for most. You’ll see a lot of guys with stats going back years who have successfully grown in length but haven’t made it past .25 inches in girth.
It’s normal to experience a little bit of growth very quickly in the first month—especially length—from the ligaments stretching and improvements in EQ. However, a lot of other guys—like me—don’t grow at all for a couple months as they start to figure out the technique. Once the real gains start, they come quickly for a few months, then taper off as your tissues becomes conditioned.
I think .25–.5 inches of girth gain is a pretty reasonable answer for what might be possible in 6 months. I’m almost to my measuring day for month 8, and I might get to record that I’ve reached .5 inches. Very few seem to grow past 1 inch in girth. However, I do think that number is doable, but hard in 2 years. Any gains past that will be very slow.
You said the right word, which is “marathon.” I think these numbers are pretty reasonable, but you shouldn’t mistake them as targets. Start slow. Do the newbie routine. Use constant heat. Try to improve your general fitness and health. Avoid all devices for 3–6 months. (Better to wait until you stop growing from manual exercises, if you can.) Read. Read. Read. Take good measurements now—photos, too, if you can. Make a progress report thread so that you can get feedback from the veterans.
Here’s an analysis of TP user stats.