It's halfway to Tigre.
You can take a train from Retiro or Chinatown in Belgrano to get there.
"How far is it?"
In Texas, we measured distance by miles.
In California, we measured distance by time. (Thus, the distance varied with traffic.)
San Isidro is about 30 minutes to the Retiro train station. Probably about 20 (or less) to the Chinatown stop in Belgrano.
In good traffic, it might be 15 to 20 minutes from Recoleta on Av. del Libertador. In bad traffic it could be 40 minutes maybe.
It's considered a very nice suburb. The people are educated and wealthy. If they don't have to, I doubt many of them come into BsAs every day. It's a commuter town for the husbands who work in the city, but the kids stay out there for their private schools. (Some kids even travel out there if they go to the right private school.)
It's technically "Gran Buenos Aires". It's not in Capital Federal (or Ciudad Autonoma de Buenos Aires- C.A.B.A.). It's cleaner, it's nicer, and it's safer than the city... and the people are better looking.
Depending on what you are looking for, you might have hit the jackpot.
Do you have a family or are you single?
How old are you?
What are you looking for?
This can determine whether the San Isidro is something that you'd want.