Complex sentence: Szpilka later admitted that at that time he "realized just what (he) had lost and began to appreciate boxing more", using the time spent in solitary confinement to work on his upper body strength and improve his boxing abilities.
Split into simpler sentences: Szpilka later admitted that at that time he "realized what (he) have had been lost, than he began to appreciate boxing more". While in solitary confinement, he devoted himself and spent that time to work up his upper body strength and improve his boxing abilities.