The space also moves
You can know a technique and still find that it arrives too late, falls short or leaves you badly positioned. Distance changes while both people move. It is something to read continuously, not a fixed mark on the floor.
Before asking how hard or fast to act, ask whether you are in a position to act at all. That question changes the way you watch a round.
Reach is only one part
Arm length alone does not describe a useful striking distance. Foot position, balance, timing and the movement of the other person matter too. A study of the lead straight punch in boxing examines the contribution of the lower limbs: a reminder that a punch is a whole-body action.
The practical lesson is not to stretch farther and lose your balance. Work with your instructor on arriving in a position from which you can move again.
Watch without rushing
A partner stepping closer does not automatically mean you should attack. Notice the rhythm, the available space and your own posture. In controlled practice, your instructor can help you recognise when to enter, when to leave and when to stay patient.
Choose a simple task for a round rather than trying to solve everything at once. Ask for feedback on one moment when you felt crowded or unable to reach.
Different arts, different contexts
Boxing, Karate and Wing Chun do not use space in exactly the same way. Rules, permitted actions and training methods change the problem. A useful idea from one discipline still needs to be tested in the context of another.
Keep technical practice supervised and agree the intensity with your partner. Good distance work depends on cooperation as well as attention.
Progress can be quiet
Sometimes improvement is a smaller unnecessary step, a calmer exit or noticing a change before it surprises you. Those moments may look less dramatic than a powerful strike, but they are part of becoming more deliberate.
Fighters draws on that patient work: respect for the art, attention to detail and the willingness to practise again.