1. Apply for Chase United MileagePlus® Explorer Card and you can get another 30000 united miles.
2. You can transfer your chase sapphire preferred points to your united mileage account and combine them together
3. 70000 united miles should be enough for one ticket
For meeting the minimum spending requirement as fast as possible, you can buy visa gift card at CVS or safeway: $500 visa gift card for $4.95 or $5.95 fee.
And you can save the visa gift card for future spending.