I've had Danes for most of my life. My 140 lb female Dane is asleep on her bed right now.
Honestly they're one of the better apartment breeds out there, especially as adults or senior dogs. Most of our Danes have lived to 8 years, with our last two (adult adoptees at 4 years and 8 years old respectively) living to 10.
They are lovers. They will sleep on the couch. You will have glasses of wine and cans of soda knocked off your coffee table by their tails. They will back up and sit on your lap, apparently completely oblivious that most of their body is still on the floor. You and your male friends will take a tail whack to the nads at some point.
The fact that you can get home midday to take the dog for a short walk will probably work out well as long as your Dane is older than 2. They can hold it, and are usually happy to sleep for two four-hour chunks back to back. A longer walk in the morning and evening would probably make them pretty happy. My folks have a four year old dog and a big yard, and she basically never spends any time outside, by her choice. You let her out, she goes and then comes and whines to be let back in so she can follow them around or sleep.
Do plan for the medical costs and the shorter life. Most of our Danes have been quite healthy, but when they get sick they are expensive simply because of their size.