A bond and a bond mutual fund are totally different animals. With a bond, you always get your interest and principal at maturity, assuming the issuer doesn't go belly up. With a bond fund, your return is uncertain because the fund's value fluctuates. When buying individual bonds, stick with new issues when possible. You get them wholesale.
Older bonds are more dicey.
They trade on the secondary market, and their prices include a dealer's markup. Don't invest all your retirement money in bonds. Inflation erodes the value of bonds' fixed interest payments. Stock returns, by contrast, tend to keep pace with inflation. Young and middle-aged people should put a large chunk of their money in stocks.