Speed

Unencumbered movement across open ground in combat is 24 meters (12 hexes at our usual map scale).

More specifically, the Move minor action lets you go 24 meters.

You can sprint at twice your current Move rate, but that increases any attacker's chance to hit by 20% (melee attacks only).

This is the Run major action.

Encumbrance (see below) and terrain will affect the distance you can move.

Over Snow

From FM 31-70, Basic Cold Weather Manual.

Encumbrance

Unencumbered

Characters carrying not more than their STR x 1.5 kg have these effects:

Move action in combat: 24 meters per round

on roads or footpaths: CON x 5 kilometers per day
cross-country: CON x 2 kilometers per day

Loaded

Characters carrying more than STR x 1.5 kg are Loaded, and apply these effects:

some skills are halved: Dodge, Climb, Jump, Swim, and other movement-related skills as the referee declares
Move action in combat is 18 meters per round

on roads or footpaths: CON x 4 kilometers per day
cross-country: CON x 1.6 kilometers per day

reduce STR by 1/4 for opposed tests

Burdened

Characters carrying more than STR x 3 kg are Burdened, and apply these effects:

many skills are halved: Dodge, Climb, Jump, Sneak, Throw, all melee attacks, all parries, and others as the referee declares
some skills may not be possible: Swim without flotation aids, for example
Move action in combat is 12 meters per round

on roads or footpaths: CON x 3 kilometers per day
cross-country: CON x 1.2 kilometers per day

reduce STR by 1/2 for opposed tests

Overloaded

Characters carrying more than STR x 6 kg are Overloaded, apply these effects:

many skills are halved: Martial Arts, Sneak, Throw, all attacks and parries, and others as the referee declares
many skills are not possible: Swim, Climb, Jump, Dodge, and others as the referee declares
Move action in combat is not more than 3 meters per round

on roads or footpaths: CON x 1 kilometer per day
cross-country: CON x 0.4 kilometer per day

reduce STR by 3/4 for opposed tests

Keep in mind that "not possible" skills may still succeed on a roll of 5% or less. These rules don't take into account training, footwear, terrain, hydration and rations, nature of load and how carried, magic, psychic powers, medical aids, gravity, etc..

Training: troops trained to carry heavy loads, and kept at that level of training, can carry up to 10-20% more weight. Note that this training is separate from calisthenics, weightlifting, fitness training, (unencumbered) running, etc. It's actually pretty simple to simulate: have you been trained in heavy load carrying, and have you been making regular loaded ruck marches to "keep in shape"? A heavy load is probably at least STR x 3 kg.
Weather: hot weather will increase the "perceived weight" of your load, up to twice its actual mass for the hottest likely weather.

And the level of exhaustion during the rest periods, strenuous combat or manual labor before or during travel, how many days the rate can be kept up, etc. are beyond any simple table of rules.

encumbrance levels, kilograms
STR	normal	Loaded	Burdened
4	≤ 6	7 - 12	13 - 24
5	≤ 7.5	8 - 15	16 - 30
6	≤ 9	10 - 18	19 - 36
7	≤ 10.5	11 - 21	22 - 42
8	≤ 12	13 - 24	25 - 48
9	≤ 13.5	14 - 27	28 - 54
10	≤ 15	16 - 30	31 - 60
11	≤ 16.5	17 - 33	34 - 66
12	≤ 18	19 - 36	37 - 72
13	≤ 19.5	20 - 39	40 - 78
14	≤ 21	22 - 42	43 - 84
15	≤ 22.5	23 - 45	46 - 90
16	≤ 24	25 - 48	49 - 96
17	≤ 25.5	26 - 51	52 - 102
18	≤ 27	28 - 54	55 - 108
19	≤ 28.5	29 - 57	58 - 114
20	≤ 30	31 - 60	61 - 120
21	≤ 31.5	32 - 63	64 - 126
22	≤ 33	34 - 66	67 - 132

Morrow Project Movement