How did General McClellan move his troops to Virginia?

By boat.

George B. McClellan claimed that the land route had been tried by General Irwin McDowell in 1861. McDowell had been defeated at the battle of Bull Run on 21 July 1861.

McClellan wanted to try a different tact. He put some 121,000 men on boats, sailed them down the Potomac River into the Chesapeake and landed them on the Virginia peninsula between the James and York Rivers. McClellan believed he could march on the Confederate capital at Richmond and capture it before the Confederates, still at Bull Run, could fall back in time to defend it. He would capture Richmond and end the war without firing a shot.

But McClellan was overcautious and constantly thought he was outnumbered, though he had an advantage of more than two-to-one for much of the campaign. He moved so slowly that the Confederates were able to fall back and protect Richmond. After Robert E. Lee attacked McClellan for seven consecutive days, McClellan retreated back to Washington. He blamed his defeat on the Lincoln government, which he claimed never gave him the number of troops he needed.