Mac's and a mixed n/ac network

That depends on the overall signal, S/N ratio, and interference and not directly the physical distance to each base station.

 

There are a number of work arounds if you only want the MacBook to link with your 802.11ac router, including the quick and dirty option of just blocking your MAC address from any other routers in the house, however the answer to your question is no not necessarily.

Your MacBook will select whichever router has the best signal strength on your roaming network, distance is not a factor in the matter.

The only way you're going to get around the wireless extension, and keep the signal without blocking yourself from the other router is to have another 802.11ac Airport Extreme base station. The downside with blocking yourself is that you will effectively reduce your wireless signal in half.

 

I made it easy on myself and for my employees. I've got an AC AE - for some reason my Macs will choose the 2.4GHz N network (-29dBm, transmit 300Mbps) over the 5GHz AC network (-59 dBm, transmit 1300Mbps) after rebooting the router 9 times out of 10 and after rebooting a Mac 3 times out of 10 - the 2.4GHz appears to be the strongest "signal", but not fastest by a longshot).

I named my two networks differently - "Fast N" and "Faster AC", and prioritized the AC network in Network Preferences (in the admin account on the Mac). QED, for me.

FWIW, we also have a few 4th-Gen AEs and a couple of 1st-Gen Express units on our network - naming the networks keeps it easy for us to choose the "weaker" but far faster 5GHz networks.