You have had already here a good mix of pros and cons - it' basically city lovers v city haters. I lived in NYC for 6 years in my 20's and it was a great start for my career. For me the city always just had an amazing energy - I took subway and walked to work, and just rented a car on weekends or subbed one out for a taxi from the airport when I had a business trip to use for the occasional get away weekend. (Jersey shore, poconos, catskills, hamptons all about 2 hours away) If your work has a shower, or you join a gym near your office, you can even run or bike to work. Anyway that energy, or buzz just really kept me going, it's there in the morning, on your lunch break, when you leave your office at night.
Nightlife and entertainment - if you like live music, ballet, opera, symphony, broadway art you can't run out of things to see. Same with sports - basketball, hockey, baseball, football, soccer - just not much playoffs LOL. As pointed out it can get expensive, but the financial pain can be limited by enjoying 1-2 quality events per week, rather than going for 7. There are also plenty of inexpensive ways to spend time as well - cheap ethnic eats, exploring new neighborhoods, dive bars with a band, cultural/neighborhood festivals (or watching high school track at the armory).
Girls are everywhere, the ratio feels like 8-2 women to men, and although everyone has their psycho avoiding defensive posture, it's actually pretty easy to crack as everyone is always looking for someone to hang with in NYC. While there is quantity, there is also quality as the city tends to draw both the best and the brightest of US college grads and maybe even more interesting - talented and adventurous foreigners trying to make it in the big city.
Running - central park is great - parks along both East and Hudson are OK. North of GW bridge around cloisters good. Subway to Van Cortlandt part excellent. Train to Roosevelt preserve near Sleepy Hollow amazing - almost limitless choice of 10-13 milers on hilly trail terrain. For cycling anything NW of Tappan Zee Bridge up into the Catskills is amazing with little traffic.
Good luck - stay out of trouble. If you wake up in the arms of a german shepard, next to a broken coffee table, with your head resting on an open garbage back of coke, you'll know you're pushing a bit too hard.