Jesus calls us to take up our cross daily and follow him. The early church continued daily in the temple and the Lord added to their number daily. Paul said, "I die daily." We are commanded to exhort one another daily. Jesus taught us to pray, "Give us this day our daily bread." He promised to be us "all the days."
Likewise, as you work out, day by day nothing visibly changes. But when you look back months or years later, everything is completely different.
God offers us grace for the grind. Nowhere do we need it more than in the "daily." The grind may go unnoticed, but the results won't.