Advancement as happens in most other RPGs is very limited in classic Traveller. Improving abilities and skills happens during downtime in much the same way as during character creation: in blocks of 4 years. From Book 2, pages 42 and 43:
Limited personal development and experience is possible in the sense of increasing abilities and skills. Such potential for increases is possible in four specific areas, only one of which may be attempted at one time: education, weapon expertise, other skills, and physical fitness.
In each field, the character selects a four-year program of self-improvement, dedicating his or her endeavors in something like obsession, with the general goal of self-improvement. Because individuals do not always have the will to continue with such a program, there is the chance that the program will be planned, but never actually carried out. After the general field has been chosen, the character must make a dedication die roll.
A low intelligence actually aids characters seeking physical fitness improvement:
Physical Fitness: Because many individuals find a regimen of physical conditioning unrewarding intellectually, a dedication throw of 8+ is required (DMs of +2 if intelligence is 8-, and 4 if intelligence is 5-). If the throw is achieved, the character increases his three physical characteristics (strength, endurance, and dexterity) each by 1.
8+ is a 41.67 percent chance of success. I find this particularly amusing given my recently mentioned desire to start a fitness blog. (For the Traveller-uninitiated, DM stands for dice modifier.)