What do we know about the diet and fitness training of soldiers in the Roman Empire? How “fit” would professional soldiers have been 2000 years ago, both in the Roman Army and in the armies of its neighbours?

For example, I’ve read that gladiators often fattened up before fights to help insulate themselves against dangerous cuts. Was that also common practice for professional soldiers, or did they not need to worry about that because they had better armour? I’ve also heard that the Roman army trained with weighted weapons to build strength; did they train for cardio as well or was that just a side effect of wearing armour and long marches? Would soldiers have been expected to be able to run long distances, or is distance running a more modern thing?

In some historical fiction I’ve read/watched, “barbarian” tribes are described or shown to be bigger and stronger than Roman soldiers. Was that actually true in some cases, or is it based on exaggeration in ancient sources? If it was true, was it due to diet (access to different foods, maybe)?

In summary, did anyone in the ancient world actually have abs like the Spartans in 300?