Related Clues:

• Double shifts at work, e.g
• Long, boring tasks
• Tedious tasks
• Tough goings
• Laborious undertakings
• Arduous efforts
• Tedious tasks ... or, parsed differently, the ends of 23-Across, 28-Down and 30-Down?
• Periods of tiring work
• Moves through mud
• Exhausting treks