Arduous

From Latin arduus ("lofty, high, steep, hard to reach, difficult, laborious"), akin to Irish ard ("high").

Full definition of arduous

Needing or using up much energy; testing powers of endurance.The movement towards a peaceful settlement has been a long and arduous political struggle.

2012, May 5, Phil McNulty, Chelsea 2-1 Liverpool, Chelsea survived and can now turn their attentions to the Champions League final against Bayern Munich in Germany later this month as they face an increasingly arduous task to finish in the Premier League's top four.

(obsolete) burning; ardentWhere flames the arduous Spirit of Isidore. — Cary.
Difficult or exhausting to traverse.

1974, Sue Bowder, The American biking atlas & touring guide, page 77:Beyond the river, an arduous slope rises 3286 feet in 13 miles.
1999, Scott Ciencin, Mike Fredericks, Dinoverse:Mike looked up from the arduous mountain trail. They'd been climbing for five hours and he was beginning to feel irritable.
2006, Jack W. Plunkett, Plunkett's Entertainment & Media Industry Almanac 2006:Survivor reaches as many as 28 million viewers who watch contestants win a new Pontiac or guzzle Mountain Dew after scaling an arduous cliff.