They had to be quick, the expense alone must have been crippling and although the
Dowager-Duchess Margaret was wealthy in her own right, the purse wasn't bottomless.
The timing was excellent though, Maximilian had just brought Bruge and Ghent to heel from their most recent rebellion and unemployed mercenaries would have been a problem. As noted above, Schwartz wasn't a nobody, he was a fairly prominent captain in Maximilian's army and within the inner circle enough to carry Max's ceremonial sword on occasion. The Irish lords of the Geraldine faction cheerfully signed up, bringing their own 'private standing armies' with them.
The claimed 200 mile march, from the West Coast, over the Pennines to Tadcaster, in five days would be pretty impressive, if it wasn't actually just 118 miles, or about 24 miles a day. This was roughly the same distance/time taken by Edward IV when he marched to Cirencester from London in 1471, albeit over flatter terrain. Assuming the lead elements set out at dawn and the last elements made camp 12 hours later, they averaged 2 miles per hour.
What I find most interesting (besides the battle itself of course), are the five days of skirmishing that took place after the Pretender's army reached Yorkshire. The Wikipedia
article fills in the details.