I'm not lonely at all, in fact I really enjoy my ability to be alone when I chose to. On weekdays after work, getting home and being done with the day, just me, my PC or TV is something I actually love. I don't know if I would have the energy to be around someone everyday. On weekends I have my friends to do stuff with. And I can go on vacations with my friends or alone. Traveling alone is really something I can enjoy, you meet a lot of people that way and the only limiting factor on where to go and what to do is yourself, although I can understand that this is not for everyone.

To your second question:
The biggest change is probably with my whole circle of friends. Nearly all of them are married now, most have kids. That changes the kind of activities you can do with them. An evening of board games or meeting up for a coffee (or a couple of drinks) always goes. Going out all night and get blackout drunk - not so much.

Relationships is a double edged sword at this age. It seems to me that at my age (33) I'm at the height of my attractiveness. I look like a man now and not like a boy, I'm fitter than ever and have a well established career. Getting dates has never been easier.
On the other hand though many women that are left on the marked at that age think they don't have much time left to get married and have kids so you thin out you pool pretty quick if you make it clear early that you are not interested in marriage (or even living together) and you really don't want to have kids. But some of my older friends tell me this will get better again once you reach you 40s and the divorcees get back into the game.