Think about when you first wake up in the morning and STREEEEEEETCH, right? It feels really good, almost like you are extending just a little bit outside the bounds of what your body is capable of. Ordinarily it's this sort of warm, relaxing, relieving, and pleasurable extending of yourself, right? In my opinion, getting an erection feels the same way, except that it's concentrated and intensified by being focused on your penis.
The process is different. When you stretch, it's a very smooth sort of motion. But an erection happens in little twitches. Usually to the beat of my pulse, but my penis twitches a bit, it feels good so it twitches a bit more, then a bit more, and each time it does it gets just a little bit longer and fuller. Eventually, these little twitches have felt better and better, as it gets bigger and bigger, like an exponential growth of pleasure until my penis is fully extended - at that point it feels just like when you've stretched as much as possible, except instead of just lasting an instant, it lasts the whole time I'm hard. More than that, every little twitch of my penis after that point is just as good as that maximum stretch.
Now imagine that you stretch at like 6:00 pm. It's not the morning, it isn't the first time you've moved for the day, but it's still a good feeling, but happens a lot faster. Getting hard faster feels like that. It's the same feeling, just accelerated. The key difference being that at maximum penis extension, it's just as amazing as a slower erection - perhaps even slightly more pressing because of the accelerated growth rate. Like the difference betwen rubbing your hands together and clapping.
Think about that tingly feeling you have in your nose just before you sneeze. Amp that up about 10 times, and that's probably about the level if good tingly feeling that a penis has at maximum, like rock hard, totally extended, throbbing erection. And it does that on a pulse. It's absolutely an amazing feeling getting hard, feeling this little piece of me grow to be such a comparatively huge thing, and to feel so absolutely incredible.
Then of course, the precum starts, which is sort of like drooling on your pillow in your sleep, but warmer, slipperier, and making the entire cock head feel so intense it's almost like you can't take it, except that you actually must and in fact need more of it.
I think that's probably about the best I can do to describe what it might feel like in terms that a woman might begin to comprehend (not that you couldn't understand per say, just that without having a penis, you just don't know the feeling). I've heard about it being similar to getting hard nipples, but men get hard nipples too, and that's really not even close to the feeling of getting a hard penis. But I wanted to try a little to put it into common terms, since just a description, in my opinion, doesn't really get the ideas across properly.
So let me know if you have more questions - this is probably as fun to describe as it is for you to read about! Suffice it to say, I love how my penis works - it's an amazing piece of human anatomy!