Maximal legspan contain lateral and vertical movements such as vertical splits and lateral splits. Rarity of people achieve sufficient technique and strength to maintain different splits and rotational movements of thighs so they are hard-to-use benchmarks.
Many factors contribute to legspan. Factors contain core flexibility, core strength, pelvis tilt and lateral hamstring movements. Fractures in vertebrae can lead to kyphosis i.e. dowager's hump (Life Span Motor Development, p.54, 2nd ed) that can stress muscles such as psoas muscle worsening movements of leg. The postnatal development contain rapid development of supranals (adrenal glands) until about 20 years old and uterus. Also the fat distribution from core and extremities change from adolescents to maturation.
The range of leg motion changes over time. Maturition contain progession while the old age contain regression. Hence the patterns of motion change. Life span motor movements in legs contain learning different gaits such as walking, running, skipping and galloping. Running is a symmetric locomotor skill while galloping is asymmetric. Running is more energy efficient than galloping because less need for stability in waist than with galloping.
Simple ways to assess leg span contain ruler. Measurements should assess the proportions of different limbs. Also as mentioned in the question, the core twisting should be acknowledged. A more straightforward way to assess running can be muscle mass analysis. Because advanced runners have more muscle mass in legs, it makes it natural to assess it directly with different resistance measurements such as InBody measurements. The impact of a single legspan characteristic can be quite meaningless.
A beginning runner has a limited range of motion. The arms and legs swing to the side rather than rhythmic forward-backward.
The range of motion develops over age (p.128-129, Life Span Motor Development 2ed, Kathleem Haywood). The examples contain the beginning runner and the advanced runner. Running is more advanced motor skill than let say walking requiring sufficient legspan for advanced technique.
An advanced runner has larger range of motion such as longer stride length, eliminated lateral leg movements, smooth strike with heel first to forefoot, limited out-toeing (second child picture on the left has excess stress to knees, bad) and better trunk rotation to allow longer stride. The arms in 90 degree swing in shoulder cups with the torso leaned slightly forward while the core is straight along the leaned axis (back leg to head). The weight comes over the support knee which requires strength and flexibility particularly from core.
Olympic-level runner Moses Mosop here has running specs:
fast backleg forward
0.133-0.167s ground contact
no bending in waist
32 steps per 10 seconds (almost 200 spm)
speed even at 3 min per km
where the running style has high cadency like in gliding running style and long stride like in gazello running style. This technique is extremely tiresome. For more typical gazello running and gliding running see the comparison here between triathlonists. The legspan requirements are more demanding in the gazello running style.
Running comparison to galloping
Galloping is favored by four-limb animals for fast movements while two-limb animals such as people do not favour them "gallop metabolically more expensive and involves high muscular stress at the hips" (Biomechanics of human bipedal gallop: asymmetry dictates leg functions) with people.
The transition from running to galloping requires about 6 months after learning running. The galloping has opposite motions between legs and arms similarly to running. A difference is that the ground times for strides are longer in galloping than running. The asymmetry of galloping means that it requires more stability from the core than running with people.
The galloping of child can be seen on the page 143 of the book with opposing arm and leg motions. In comparison, the galloping is similar with horses where the arms act as supports so less stress to waist than with people. This is the "first asymmetrical locomotor pattern a child learns" (p.142).
Summary and motivational areas
Interesting areas for further research contain the assesssment of biomechanics on different running styles. At the time of writing, the author does not know whether different legspan characteristics have been analysed for running. A simple asssessment methods contain ruler methods and muscle mass analysis.