The area you're interested in is more commonly referred to as Tripolitania and Cyrenaica in current literature, although Libyans are also sometimes referred to. The best answers to your question could probably be found in Leslie Dossey's Peasant and Empire in Christian North Africa (2010), or in her footnotes. Dossey includes Libya in her discussions of North Africa, although when many other scholars refer to North Africa, they seem to mean only the areas of modern Tunisia and Algeria.

Dossey argues that North Africa prospered during late antiquity and increasingly adopted the culture of romanitas and put it to local use. She's especially interested in tracking Christianity (which included Catholics, Donatists, and - I would argue - Manichees), the spread of bishops into the countryside, the use of Latin, and the spread of fineware pottery.

The "Third-Century Crisis" and the Vandal Conquest (430s) may have actually benefited North Africa, since they were no longer burdened with sending tremendous taxes to Rome. That meant more food could stay at home. Locals now had the surplus to take time off from agriculture and try working at crafts, which was a necessity now that North Africa had severed its ties with imperial tax and trade networks. In practical terms, this meant that antique Libyans ate more, had better access to quality goods, prided themselves on their civilized Latinity, and gathered themselves around an increasing number of local bishops, who brought communal justice and identity down to the level of the most meager estates.

Sources are few for how far these changes actually reached into Libya, but Dossey notes that Libyans were still etching their epitaphs in Latin in the year 1150 -- 500 years after the Islamic conquests!

As regards the travel time between settlements, I'd point you toward Michael Decker's Tilling the Hateful Earth (2009). His focus is on the province Oriens (roughly modern Israel and Syria), but he does discuss the east Mediterranean more generally, and his picture of the rural landscape seems appropriate for comparison.

Decker argues that there was an intense demand for land during late antiquity, which is especially surprising given unfavorable climactic conditions. Nevertheless, our stalwart late ancients undertook massive irrigation projects, sometimes going so far as to create artificial oases deep in the desert, all to get some oil or wine to exchange in the still-thriving east Mediterranean markets. He paints a vivid picture of the landscape:

"A sixth-century traveler who journey overland from Bostra to Antioch would have traversed a patchwork landscape. The traveler would have passed through many of the close-knit, black basalt villages, their houses looking in on one another rather than out to the fertile plain. Beyond the villages lay the carefully tended gardens and orchards that broke into the open grain fields. Our traveler might be in lonelier country, but not an empty one: outside the villages lay scattered farmsteads, military posts, way-stations, estate houses, and monasteries." (p. 44)