I am a Mexican distance coach living in Mexico City.
There are a lot of safe places to run in Mexico City, just to give you an idea check this site:
http://www.endondecorrer.com/#
!distrito-federal/czsp
Here Mexico City is known as DISTRITO FEDERAL or D.F.
For long distance runs on dirt paths and hilly course we have Bosque de Tlalpan (south):
http://www.endondecorrer.com/#
!bosque-de-tlalpan/c1nsb
For some fast tempo runs or long repetitions on flat surface we have a beautiful place called Viveros de Coyoacan, besides just outside the park you can buy a very nice coffe!
http://www.endondecorrer.com/#
!viveros-de-coyoacan/c1dj9
These two places are full of recreational runners from 5:00 AM until late afternoon. Of course that there are some women athletes!
Worried about traffic and pollution? Nobody has died so far from pullution yet! which is very similar like Los Angeles, Athens or Madrid, I have been in these cities and all of them have similar problems regarding crime, pollution and crazy taxi drivers.
The average rent and food in Mexico City is cheap compared with other cities in Mexico, besides the food is more natural than the US or some European countries, we have less fat people!
After your training run in the morning you can buy a natural orange juice just made in front of you, something that an athlete from Europe can only watch on TV or video!
Why the Mexican distance runners are not as good as in the 90's? It is a very long answer because there are many factors.
There is poverty in Mexico but you can not compare it like the one that exists in Kenya or Jamaica. Here the average young athlete in Mexico City, Puebla, Guadalajara or Monterrey (the major cities) goes to school/University and always has several options besides training or being professional athlete.
Altitude is just one factor in any training system, living and training at altitude may help you, but it would be more helpful and important if you can join a distance training group guided by a professional Coach.