Here are some proposed rule mods for movement and actions in combat for Traveller. I've based the numbers on reasonable human values, but also tried to make it work out similar to what everybody is used to in D&D. The main difference is that, during combat, characters can't move AND attack on the same round. This shortens the time between player turns and I think makes combat play out in a more realistic way (e.g. you can't run up to someone and club them without giving them a chance to run away). If you think about it, we're really just splitting normal "D&D" type rounds into "half-rounds" (and then calling them full rounds).
Movement
Up to encumbrance limit, a character's movement speed is 2 m/s (7.2 kph) when walking and 3.33 m/s (12kph) running. Beyond encumbrance, speed will be decreased. Characters may "sprint" at double speed but consecutive sprints are not allowed.
As usual we won't be tracking movement and time too closely during normal game play (exploring, traveling), unless it matters for some reason.
Movement and Action in Combat
Here are some basic rules for movement and action in combat:
Combat "rounds" are 3 seconds
In combat situations a character may take 1 Major Action (e.g. attack, move, reload, heal, etc.) and 1 Minor Action (open unlocked door, verbal communication, etc.)
A character may move 10m in one round at running speed ( 3s x 3.33m/s = 10m)
A character may move 20m in one round at sprint speed (3s x 6.66m/s = 20m)
A character may move less than 10m or 20m in one, forfeiting the remaining movement.
A character moving between two "cover" objects (rocks, buildings, etc.) is susceptible to ranged attacks of opportunity if the distance between the cover objects is greater than 1/2 the move base distance (i.e 5m when running or 10m when sprinting).
The round duration and running speed were chosen to result in the 10m/round move distance (Item 3). This is pretty close to the D&D 30ft. default.
Per Item 5, you can always move less in your allotted 3 seconds, but if so you are considered to just be moving slower. The short duration of the round doesn't really allow for splitting up the move. That said, your minor action could be done "during" movement. For example you could toss a previously armed grenade while running or sprinting (although at reduced accuracy).
Item 6 just means you can't run out in the open without potentially getting shot at.
(reference links: The Soldier's Heavy Load, The Soldier's Ideal Speed)