What is the best way to remove old linoleum from hardwood floors?

You need to find out if it has asbestos in first. If it's 20 years it probably does. If it does wear protective plastic clothing, goggles, gloves, hat or hood and mask. Make sure to bag flooring as you remove it. Cut in sections and pull up and bag then move to next section and so on. If floor has been glued pour boiling water on floor until glue softens then use plastic paint scrapper to scrape glue from floor. Check this link for more info.

https://www.todayshomeowner.com/how-to-remove-glued-down-linoleum-or-vinyl-from-a-wood-floor/

Best not to do above..... you could spew asbestos fibers throughout the whole house. Most floor people recommend if it is asbestos flooring to put a thin plywood sheathing down and install new floor. If old lino is smooth you might be able to just install over it if your doing a full sheet.

IF you decide to remove the glue [as stated by MS Texas], then be very careful INCASE of asbestos...if there is no worry about asbestos, you can sand whatever you couldn't get up off the wood. I would sand the wood & beautify it with sealer & paste wax & buff it until the day I died...enjoying it every day! But that's me.