past awhile > [when some time had passed] season > occasion; opportunity
2 To leave that desert mansion, she cast
desert > lonely mansion > lodging, place of abode cast > resolved
3 In secret wise herself thence to withdraw,
wise > manner
4 For fear of mischief, which she did forecast
mischief > evil forecast > conjecture
5 Might by the witch or by her son compassed:
compassed > [be] contrived
6 Her weary palfrey, closely as she might