Another vote for intervals. However, I need to take my own advice since after my back injury, I have not been doing any. Speed is there, but the distance at that speed is getting shorter and shorter. It is a lot of work, but it gets results. Just don't expect it to be immediate.
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HCFR Cycling Team
Ride Safe ... Ride Hard ... Ride Daily
2017 Colnago C-RS
2012 Colnago Ace
2010 Giant Cypress