Take the other half of your yeast vial and make a yeast starter.
Use a pint of water and about 1/8 lb of agave. Make sure to add a little yeast nutrient.
Side note: I'm estimating on the 1/8#, based on the ratio I use in my mead starters. Agave is a little lighter so you may need some more. Shoot for an OG of around 40 in the starter.
You may want to double the pint to a quart, but I figure at your 2 gal volume you should be OK with a pint.
Ex fermentum, scientia.