Wiktionary
prep.phr. Not fit. Not healthy enough to do any hard, physical work or sport.
WordNet
adj. not in good condition
Usage examples of "out of condition".
Even though the cat was old and lean and out of condition, scarred by years of hunting in thorn veld and his coat dull and shaggy, yet his belly was crammed with beef and he would weigh four hundred pounds or more.
Grossly out of condition, like ninety-nine per cent of men to-day.
Spare tents and furs were either missing or badly out of condition.
Masinissa was at Gades, and on being informed by Marcius that Scipio was coming, he represented to Mago that his horses were getting out of condition through being confined in so small an island, and were causing a general scarcity from which all alike suffered, whilst his cavalry were becoming enervated through inaction.
Not only were we all out of condition, but the slightly heavier gravity was telling.
Because you're not used to it, it'll be a fairly lazy pace, but it's a long time for someone out of condition on horseback to spend in the saddle.