Normal progression would be
Howling Mountains > Silverspring > Aslan Valley > Ystra Highlands > Dust Devil Canyon > Ravenfell > Weeping Coast > Savage Lands > Aotulia Volcano > Thunderhoof Hills > Southern Janost Forest > Northern Janost Forest > Limo Desert > Land of Malevolence > Redhill Mountains > Tergothen Bay.
There are two other chains to follow
2nd class: Sascilia Steppes > DragonFang Ridge > Ystra
3rd class: Coast of Opportunity > Xaviera
As far as levels, this would be good rule of thumb:
Howling Mountains (1-10) > Silverspring (10-20) > Aslan Valley (20-30) > Ystra Highlands (30-40) > Dust Devil Canyon (40-45) > Ravenfell (45-50) > Weeping Coast (50-53) > Savage Lands (53-54) > Aotulia Volcano (54-55) > Thunderhoof Hills (55-57) > Southern Janost Forest (57-5

> Northern Janost Forest (58-60) > Limo Desert (60-62) > Land of Malevolence (62-65) > Redhill Mountains (65-67) > Tergothen Bay (67-70).
Sascilia Steppes (1-20) > DragonFang Ridge (20-30) > Ystra
Coast of Opportunity (1-42) > Xaviera (42-51)