I'm a single 40 something and do go to the bars but I have not gone by myself. I usually go with one or two of my other single girlfriends but I gotta tell you - it's tough. First, it's really hard to find bars that have guys in our age range - far and few between. Secondly, if there are single guys our age out they are paying all of the attention to the younger girls! I actually find that there are more men out at happy hours, especially in the Tysons area, but I have yet to make a love connection. I was actually just saying today that it sucks being this age and single - it's really hard finding guys. And I am not against someone younger than I am but they don't want someone my age - maybe for a hook-up bit that's it.
If you do decide to go to a bar by yourself, I'd go to happy hour on a weeknight first and see how it goes. Go where there are a lot of businesses like Tysons. And like some of the other's said, I wouldn't stay long, get to know the bartendars.
If any of your 40 something single men are reading this please tell us where you guys go! We want to meet you