First off, in my system of Aikido (Tomiki based), we work from at least two different distances with the longer being three steps out. While this is where uke starts, the actual distance where the technique starts is inside Ma'i.
In terms of maintaining ma'i when in a live-fight situation, it's just something you have to work on and practice in order to maintain. You should never let your feet get planted but rather keep moving all the time, controling when uke crosses ma'i. This is where randori becomes very important in terms of practical self-defense training and something that should be practiced regularly with both single and multiple ukes, depending on your skill level.
Hope this helps.